FR2774469A1 - TORQUE SENSOR FOR ROTATING SHAFT - Google Patents

Abstract

The invention concerns a device for measuring torsional moment on a rotating shaft, said device comprising at least a magnetic field generator arranged in a first plane of said shaft cross-section and at least a magnetic field sensing member, in fixed position in a second plane of said shaft cross-section, the sensing member delivering a signal proportional to the torsional moment following the angular offset of the field generator relative to the sensing member, the magnetic field generator having a magnetised structure with opposed magnetising directions and is supported by support means integral with the rotating shaft control means, the magnetic field sensor being arranged substantially opposite the magnetic field generator and being supported by support means integral with the rotating shaft.

Description

 The invention relates to the technical field of torque sensors, that is to say devices for detecting the application of torque on a shaft.

 The invention relates more particularly to the measurement of torque applied to a shaft.

 To measure the torque transmitted between two rotating members, elastic torque couplers are generally used.

 The deforming element is frequently formed by a torsion bar.

 To avoid left torsion and a stress concentration detrimental to fatigue resistance in particular, the torsion bars of the torque meters are conventionally of circular section.

For a given material, in isotropic linear elasticity, the angle of torsion e is worth in pure torsion
e = 32ML
g
rD G
where D is the outer diameter of the hollow bar or not
M is the torque applied to the torsion bar
G is the modulus of transverse elasticity
L is the useful length of the bar.

 So that for a given material and bar geometry, it is possible to relate the angle of torsion to the torque applied to the bar.

Torsion bar torque transducers can be found in the following documents: FR-2 705 455, GB-2 306 641, W0-87 / 02319, WO-92/20560, WO-95/19557, WO- 96/06330, WO-97/08527, WO-97/09221, EP-325 517, EP-369 311,
EP-286,053, EP-437,437, EP-418,763, EP-453,344, EP-515,052,
EP-555,987, EP-562,426, EP-566,168, EP-566,619, EP-638,791,
EP-673 828, EP-681 955, EP-728 653, EP-738 647, EP-738 648,
EP-765,795, EP-770,539, EP-802,107.

The main methods for measuring the torque of a rotating shaft, with or without a torsion bar, are as follows
- method based on an electromagnetic phenomenon
- optical methods
- electrical methods.

 Magnetic methods essentially use magnetostriction and the Hall effect.

 By magnetostriction is meant the reversible mechanical deformation which accompanies the variation in magnetization of a ferromagnetic solid.

 This phenomenon is reversible: a deformation exerted on a ferromagnetic material placed in a magnetic field causes a variation of the magnetization (reverse magnetostriction).

Examples of magnetostrictive or magnetoelastic detectors allowing the measurement of torque by measuring the variations in permeability of the magnetically anisotropic zone can be found in the following documents: EP-229 688, EP-261 980, EP-270 122,
EP-288 049, EP-309 979, EP-321 662, EP-330 311, EP-338 227,
EP-384 042, EP-420 136, EP-422 702, EP-444 575, EP-502 722,
EP-523,025, EP-562,012, EP-651,239.

 By Hall effect, we conventionally designate the production of an electric field normal to the current density vector in a conductor or semiconductor placed in a magnetic induction field normal to the current density vector.

Torque sensors implementing the effect
Hall can be found in particular in the following documents: FR-2,689,633, FR-2,737.010.

Optical torque measurement methods are essentially associated with interference phenomena or optical density measurement. Reference may be made, for example, to the following documents: EP-194,930, EP-555,987,
US-5,490,450, US-4,676,925, US-4,433,585, US-5,001,937, US4,525,068, US-4,939,368, US-4,432,239, FR-2,735,232, FR-2,735 233, WO-95/19557.

 The electrical methods of torque measurement are essentially linked to the capacitive measurement or to a phase difference measurement between two magnetic encoders mounted circumferential to the torsion axis.

 The documents EP-263 219, EP-352 595, EP-573 808 relate to devices for measuring torque by strain gauge or strain gauge.

 The document EP-442 091 describes an installation for measuring the angle of rotation or the torque of a rotary or fixed element of a machine, comprising a torsion element in the form of a spoke wheel connected to several measuring elements, at least one spoke of the spoke wheel being cut so that the portions of the spoke or spokes are pressed against each other when moving a predetermined bending of the other spokes. The measuring device implements the eddy currents.

 The invention relates to a torque sensor for a rotating shaft, this sensor being short, of great stiffness, insensitive to electromagnetic disturbances, of simple construction, not requiring the dissociation into three parts of the shaft on which must be measured the torque.

 This torque sensor or torque meter has at least one magnetic field generator and at least one magnetic field detector, Hall effect.

 The measuring device comprises at least one magnetic field generator arranged in a first plane of a cross section of said shaft and at least one member detecting the magnetic field, in a fixed position in a second plane of a cross section of said shaft, l detector unit delivering a signal proportional to the torque due to the relative angular offset of the field generator relative to the detector member, the magnetic field generator having a magnetized structure with antiparallel magnetization directions and being supported by means support integral with the control means of the rotating shaft, the magnetic field detector being disposed substantially opposite the magnetic field generator and being supported by support means integral with the rotating shaft.

The support means of the magnetic field generator and the support means of the magnetic field detector are formed by
- a first deformable outer ring
- a second, substantially unstressed, outer ring placed at a distance from the deformable outer ring,
the deformable outer ring being able to be rigidly secured to the means applying the torque to be measured on the driven shaft,
the deformable outer ring being assembled to an inner ring by at least one elastically deformable means, the inner ring being integral in rotation with the driven shaft,
the substantially unconstrained outer ring being assembled to the inner ring by at least one substantially unconstrained means,
so that the relative angular offset of the field generator relative to the field detecting member is caused by the small relative displacement of the two outer rings, one with respect to the other, under the effect of the torque applied by the means applying the torque.

 In one embodiment, the magnetic field generator is supported by the deformable outer ring, the field sensor being supported by the substantially unstressed outer ring.

 In another embodiment, the magnetic field generator is supported by the substantially unstressed outer ring, the field sensor being supported by the deformable outer ring.

 The device comprises, in one embodiment, two field sensors and two field generators arranged facing one another over a diameter of the outer rings.

 The outer rings are coaxial and substantially the same diameter.

 The elastically deformable means associating the first deformable outer ring and the inner ring is a beam extending radially from the inner ring towards the deformable outer ring.

 The substantially unconstrained means associating the second substantially unconstrained outer ring and the inner ring is a beam extending radially from the inner ring towards the second substantially unconstrained outer ring.

 In another embodiment, the elastically deformable means associating the deformable outer ring the inner ring is a torsionally deformable tube.

 The invention also relates to the application of a torque sensor as presented above to the power steering columns of a motor vehicle.

Other objects and advantages of the invention will appear more fully during the following description of embodiments, which description will be made with reference to the accompanying drawings in which
- Figure 1 is a perspective view of a flexural test body hub according to one embodiment
- Figure 2 is a front view corresponding to Figure 1
- Figure 3 is a front view of a flexural test body hub according to another embodiment;
- Figure 4 is a front view of a flexural test body hub provided with stop beams according to one embodiment
- Figure 5 is a sectional view along the line
VV of figure 4
- Figure 6 is a front view of a flexural test body hub, according to another embodiment
- Figure 7 is a sectional view along the line
Figure VII-VII
- Figure 8 is a sectional view along line VIII-VIII of Figure 6
- Figure 9 is a perspective view of a torsion proof body hub according to one embodiment;
- Figure 10 is a front view corresponding to Figure 9
- Figure 11 is a sectional view along the line
XI-XI of figure 10
- Figures 12, 13 and 14 are front views of hubs with bending test bodies according to other embodiments
- Figure 15 is a schematic perspective view of a Hall effect sensor intended to be integrated into a hub as shown in Figures 1 to 14, according to one embodiment.

 We first refer to Figure 1.

 The hub with bending test body shown in perspective in FIG. 1 is intended to be integrated between a control means for a driven shaft and this driven shaft or between a driving shaft and a driven shaft.

 This hub 1 comprises a cylindrical inner ring 2 and two outer rings 3a, 3b connected to the inner ring 2 by elastic beams deformable in bending 4a and non-deformed beams 5.

 More specifically, the outer ring 3a fixed to the control means of the shaft driven by screws or equivalent passing through holes 6, is connected to the inner ring by means of elastic beams deformable in bending 4a.

 The outer rings 3a, 3b are, in the embodiment, substantially coaxial and of the same mean diameter.

 In the embodiment shown, the deformable beams 4a are four in number, regularly distributed perpendicular to the axis of the driven shaft D.

 In other embodiments, not shown, these deformable beams are two, three or more than four in number.

 The outer ring 3b is connected to the inner ring 2 by means of non-deformed radial beams 5.

 In the embodiment shown, these non-deformed beams 5 are the same number as the elastically deformable beams in bending 4a, the beams 4a, 5 being located substantially in two radial planes perpendicular to the axis of the driven shaft D.

 In other embodiments, not shown, the beams 5 are two, three or more than four, the number of beams 4a being four.

 In certain embodiments, not shown, the number of beams 4a is different from four is different from the number of beams 5.

 In other embodiments, not shown, the number of beams 4a is equal to the number of beams 5, this number being different from four.

 In still other embodiments, not shown, the outer ring 3b is connected to the inner ring 2 by means of an annular web.

 The beams 4a, 5 can be, as shown, substantially arranged perpendicular to each other, according to common radial planes.

 In other embodiments, not shown, the beams 5 are arranged in radial planes offset with respect to the radial planes of the beams 4a.

 The deformable outer ring 3a of the hub 1 is rigidly associated with the control means for the driven shaft, screws or any other equivalent means passing, for example, through holes 6 in the tabs 7.

 Alternatively, the test body hub may be in one piece with the control member of the driven shaft, for example made integrally with it or welded to it by any suitable means.

 When the control means of the driven shaft exerts a force on the outer ring 3a, a bending deformation of the beams 4a is obtained, deformation all the greater as the resistance torque on the driven shaft is important.

 The outer ring 3b, meanwhile, remains substantially unstressed. Its position can thus serve as a reference base for measuring the displacement of the outer ring 3a.

 The outer ring 3b carries sensors 8 capable of measuring small displacements, of the order of a few microns to a few hundred microns.

 In the embodiment shown, these sensors 8 are two in number and are arranged in axial housings 9, formed in the front ring 3b, in line with the rear ring 3a.

 The sensors 8 are Hall effect or magnetoresistive.

 Although a single Hall effect probe is sufficient for measuring small displacements, it is possible, for reasons of reliability, to have several probes in the measurement gap 10 in order to create redundancy.

 Each of the probes can have its own associated electronic circuit.

 By comparison or combination of the signals delivered by two, three or four different probes, it is possible to detect a possible failure of one of the probes and to ensure good reliability of the torque meter.

 A second embodiment of the hub 1 will now be described with reference to FIG. 3.

 The hub 1 shown in FIG. 3 comprises, like the hub which has just been described, a deformable outer ring 3a, an unstressed outer ring 3b, an inner ring 2, deformable beams 4a connecting the ring 3a to the inner ring 2 and non-deformed beams 5 connecting the ring 3b to the inner ring 2.

 In the embodiment shown in Figure 3, the hub has four beams 4a whose section varies from the foot 11 to the head 12 of these beams.

 In other embodiments, the test body comprises one, two, three or more than four beams of variable section from their foot to their head.

 This variation can be regular or not.

 This variation can be linked to a variation in the width of the beam and / or to a variation in the thickness of the beam.

 The thickness h of the beam is measured tangentially to a circle centered on the main axis D of the driven shaft.

 In the embodiment shown in Figure 3, this thickness h varies substantially linearly.

 In other embodiments, not shown, this thickness h varies in a polynomial, logarithmic manner, continuously or not when one moves away from the axis D of the driven shaft.

 The width b of the beams 4a, measured in the direction D, is substantially constant in the embodiment considered in FIG. 3.

 In other embodiments, not shown, the width b varies linearly or polynomially, the height h also being variable.

 Reference is now made to FIG. 4 which is a front view of a hub with a bending test body, provided with stop beams 13.

 In the embodiment shown, two stop beams 13 extend radially in a transverse direction T from the inner ring 2 towards the deformable outer ring 3a.

 The length L of the stop beams 13 is less than that of the deformable beams 4a, the end part of each stop beam 13 being engaged, with a predetermined clearance, in a deformation stop 14.

 The deformation stops 14 project internally from the outer ring 3a and have a groove 15 of width 1 greater than the width 1 ′ of the stop beams 13.

 The play existing between the stops and the beams 13, linked to the difference in width 1-1 ', can be determined as a function of the maximum admissible deformation for the beams 4a.

 And this, for example, in order to avoid their plastic deformation.

 Figures 6 to 8 illustrate another embodiment of the hub 1 with bending test body.

 In this embodiment, the width b of the deformable beams 4a varies from feet 11 to head 12 of these beams, in decreasing manner.

 This decrease can be linear or polynomial.

The number of deformable beams 4a, their angular distribution, the thickness and the height of the beams, the material used to make them condition as will clearly appear to those skilled in the art, the following characteristics
- inertia module
- maximum stress in beams, for a given maximum torque, for example at failure.

 The test body can be made of a material chosen from the group comprising: steels, cast irons, aluminum alloys, magnesium alloys.

 The test body can be molded or machined depending on the materials used, the geometry of the beams, the admissible cost in particular, as those skilled in the art will be able to determine.

 When the test body is made of aluminum or a magnesium alloy, it can be molded with a metal insert comprising the grooves for mounting the hub 1 on the driven shaft.

 Referring now to Figures 9 to 11 which illustrate an embodiment of a torsion test body hub.

 The hub 1 includes an unconstrained outer ring 3b, of substantially cylindrical outer peripheral surface.

 This ring 3b is provided with two housings 9, formed in two diametrically opposite extra thicknesses 16.

 Between these extra thicknesses 16, the internal surface of the ring 3b is substantially cylindrical.

 The ring 3b is assembled to the inner ring 2 by at least one beam 5, a veil or any other substantially rigid connecting element.

 In the embodiment shown, two radial beams 5, integrally formed with the inner ring 2 and the unconstrained outer ring 3b combine these two rings 2, 3b.

 The beams 5 are, in the embodiment shown, of substantially constant square cross section from their feet 11 to their head 12 and are substantially aligned.

 The inner ring 2 has a through hole defining a grooved endpiece 17 for fixing to the driven shaft and, on the other hand, a bearing surface 18 of the driven grooved shaft.

 A torsionally deformable tube 4b connects the inner ring to the deformable outer ring 3a.

 In one embodiment, the torsionally deformable tube is perforated longitudinally, openings extending in the direction D separating deformable beams in torsion bending.

 The deformable ring 3a is rigidly assembled by applying torque on the driven shaft.

 Screws or the like ensure, via the holes 6, the fixing of the hub 1 to the means of applying the torque to the driven shaft.

 When the hub 1 is rigidly associated with the application of torque to the driven shaft, the deformable outer ring 3a secured to the torque application means is moved in rotation relative to the unconstrained outer ring 3b.

 The measurement of this small displacement by means of Hall or magnetoresistive probes 8 placed in the housings 9 and magnets 8 ′ fixed on a support plate 19 secured to the driven shaft allows the measurement of the torque applied to the shaft led.

 Reference is now made to FIG. 12 which represents another embodiment of a hub with a bending test body.

 In this embodiment, the elastically deformable means connecting the inner ring 2 to the deformable outer ring 3a are in the form of coils.

 These coils form several bends 20 separated by sectors substantially in an arc of concentric circles 21.

 These coils extend substantially in the same plane perpendicular to the axis D of the driven shaft.

 The thickness of each coil is, in the embodiment considered to be substantially constant from the base 22 to the head 23 of these coils.

 In other embodiments, not shown, the coils are two, three or more than four in number.

 The thickness of at least one coil can be variable, from its head to its foot, if necessary.

 Figures 13 and 14 are front views of a bending test body hub, comprising more than four deformable beams, in this case twelve beams distributed radially regularly around the axis D.

 Starting from the test body shown in FIG. 13, it is possible, by machining or any other equivalent means, to obtain the test body represented in FIG. 14, comprising no more than ten deformable beams, four of which serve as stops, during application of a torque exceeding a threshold value.

 The stop is obtained, whatever the direction of rotation requested from the drive shaft, as soon as a threshold torque value is reached, by contact between the end portion 24 of the stop beams 25 and the internal projection of the ring non-deformed exterior 3b.

 According to the radial angular arrangement of the stop beams 25, the maximum torque admitted in the clockwise direction H may be greater, equal or less than the maximum torque admitted in the counterclockwise direction AH.

 The flexural deformable beams described above include, in certain embodiments, cutouts.

 When applying a torque, only the beams not cut into two sections transmit the forces, the sections of the cut beams transmitting a bending force only when an applied torque threshold is exceeded.

 The two sections of a cut beam are, in one embodiment, spaced from each other by a predetermined length as a function of said threshold value for the torque.

 The cutting of a beam is, in one embodiment, arranged substantially at 450 with respect to the radial direction of the beam in question.

Cutting out at least one deformable beam 4a can make it possible, depending on the number and arrangement of all the beams in particular,
-or obtain overload protection, in both directions of rotation possible
-or to obtain a torque meter with several torque measurement ranges, the rigidity of the torque meter increasing when a large number of beams are placed under load.

 Reference is now made to FIG. 15 which is a perspective view of a sensor 8 according to one embodiment.

 This sensor 8 comprises a cylindrical body 26 made of ferromagnetic material and a magnetic detector 27 intended to face the magnetic field generator such as a magnet 8 '.

 The sensor 8 comprises, opposite the magnetic detector 27, a part forming a stop 28 limiting the axial movement of the sensor 8 in the housings 9.

 The magnetic detector 27 comprises a sensitive element 29 eccentric with respect to the circular section of the sensor 9 so that the rotation of the sensor 8 around the axis 0z generates a displacement along the x axis of the sensitive element 29.

 When the sensors 8 are assembled in the factory, the operator completes the assembly by measuring the signal supplied by the two sensors 8 using a suitable device.

 This signal is a function of the position of the sensitive element 29 with respect to the magnetic transition, so that the operator can, by turning the sensor 8, bring the sensitive element 29 opposite the magnetic transition of the magnetic field generator and cancel said signal.

Once this adjustment has been made, the sensors are immobilized, for example by means of an adhesive.

 The adjustment described above is designated by the expression adjustment by offset.

 The power of the signal supplied by each magnetic detector 27 can also be modulated by modifying the axial penetration of the sensors 8 in the housings 9 so as to modify the air gap between the detector 27 and the magnet 8 'facing it.

 In the embodiment of the sensor 8 shown in FIG. 15, the minimum value of the air gap is fixed by the place forming a stop 28, coming to be pressed against the front face of the extra thicknesses of the non-deformable ring 3b.

 Calibration of the torque meter can be obtained, for example, by applying a calibrated load and adjusting the signal amplification level.

An electronic circuit associated with the test body comprises, in one embodiment,
-a current supply for the energy supply of the Hall probes
-a signal filter circuit from the probes, to eliminate background noise
-a module ensuring analog to digital signal conversion
a module for controlling and compensating for drift of the signal emitted by the probes as a function of the temperature, for example in a range -40 "+80" C
-a security module regularly testing the proper functioning of each of the probes.

 If necessary, the electronic circuit includes a module making it possible to fix the threshold for triggering the steering assistance, threshold corresponding to a determined value, or else a module for transmitting the signal wirelessly or without contact.

 The electronic circuit can be associated, for example by gluing, with the non-deformed ring 3b, on the front face.

Claims (31)

CLAIMS 1. Device for measuring a torque on a rotating shaft, this device comprising at least one magnetic field generator arranged in a first plane of a cross section of said shaft and at least one magnetic field detecting member, fixed position in a second plane of a cross section of said shaft, the detector member delivering a signal proportional to the torque as a result of the relative angular offset of the field generator with respect to the detector member, characterized in that the generator field has a magnetized structure with antiparallel magnetization directions and is supported by support means integral with the control means of the rotating shaft, the magnetic field detector being arranged substantially opposite the magnetic field generator and being supported by support means integral with the rotating shaft.  2. Torsional torque measuring device according to claim 1, characterized in that the support means of the magnetic field generator and the support means of the magnetic field detector are formed by  - a first deformable outer ring (3a)  - a second, substantially unstressed outer ring (3b) placed at a distance from the deformable outer ring (3a),  the deformable outer ring (3a) being able to be rigidly secured to the means applying the torque to be measured on the driven shaft,  the deformable outer ring (3a) being assembled to an inner ring (2) by at least one elastically deformable means (4a, 4b), the inner ring (2) being rotationally integral with the driven shaft,  the substantially unstressed outer ring (3b) being assembled to the inner ring (2) by at least one substantially unstressed means,  so that the relative angular offset of the field generator (8 ') with respect to the field detector member (8) is caused by the small relative displacement of the two outer rings, one with respect to the other, under the effect of the torque applied by the means applying the torque.  3. A device for measuring a torque according to claim 2, characterized in that the magnetic field generator is supported by the deformable outer ring, the field sensor being supported by the substantially unstressed outer ring.  4. A device for measuring a torque according to claim 2, characterized in that the magnetic field generator is supported by the substantially unstressed outer ring, the field sensor being supported by the deformable outer ring.  5. A device for measuring a torque according to any one of claims 1 to 4, characterized in that it comprises two field sensors and two field generators arranged facing one another over a diameter of the outer rings (3a, 3b).  6. A device for measuring a torque according to any one of claims 1 to 5, characterized in that the outer rings (3a, 3b) are coaxial and substantially of the same diameter.  7. device for measuring a torque according to any one of claims 1 to 6, characterized in that the elastically deformable means associating the first deformable outer ring (3a) and the inner ring (2) is a beam (4a) extending radially from the inner ring (2) to the deformable outer ring (3a).  8. A device for measuring a torque according to claim 7, characterized in that it comprises several beams (4a) elastically deformable, extending radially from the inner ring (2) towards the first deformable outer ring ( 3a).  9. A device for measuring a torque according to claim 8, characterized in that the beams (4a) are equidistant from each other.  10. A device for measuring a torque according to claim 7, characterized in that the elastically deformable beams (4a) are not equidistant from each other.  11. A device for measuring a torque according to any one of claims 7 to 10, characterized in that the height of at least one elastically deformable beam (4a) varies from its foot to its head.  12. A device for measuring a torque according to any one of claims 7 to 11, characterized in that the thickness of at least one elastically deformable beam (4a) varies from its foot to its head.  13. A device for measuring a torque according to claim 11, characterized in that the height of the set of elastically deformable beams (4a) varies identically from their foot to their head, the thickness of said beams (4a) being constant.  14. A device for measuring a torque according to claim 12, characterized in that the thickness of the set of elastically deformable beams (4a) varies identically from their foot to their head, the height of said beams (4a) being constant.  15. A device for measuring a torque according to any one of claims 11 to 14, characterized in that the height of the elastically deformable beams (4a) varies linearly from the foot to the head of said beams ( 4a).  16. A device for measuring a torque according to any one of claims 11 to 14, characterized in that the height of the elastically deformable beams (4a) varies in a polynomial manner from the foot to the head of said beams ( 4a).  17. A device for measuring a torque according to any one of claims 12 to 14, characterized in that the thickness of the elastically deformable beams (4a) varies linearly from the foot to the head of said beams (4a).  18. A device for measuring a torque according to any one of claims 12 to 14, characterized in that the thickness of the elastically deformable beams (4a) varies polynomially from the foot to the head of said beams (4a).  19. A device for measuring a torque according to any one of claims 7 to 18, characterized in that the substantially unconstrained means associating the second substantially unconstrained outer ring (3b) and the inner ring (2) is a beam (5) extending radially from the inner ring (2) towards the second substantially unstressed outer ring (3b).  20. A device for measuring a torque according to claim 19, characterized in that it comprises several beams (5) substantially free of stress extending radially from the inner ring (2) towards the second non-outer ring constrained (3b) and connecting them.  21. A device for measuring a torque according to claim 20, characterized in that the beams (5) substantially free of stress are equidistant from each other.  22. A device for measuring a torque according to claim 21, characterized in that the beams (5) substantially free of stress are not equidistant from each other.  23. A device for measuring a torque according to claim 21 or 22, characterized in that the beams substantially free of stresses (15) are substantially arranged in the same radial planes as the deformable beams (4a).  24. A device for measuring a torque according to claim 21 or 22, characterized in that the beams substantially free of stress (5) have a geometry substantially identical to that of the elastically deformable beams (4a).  25. A device for measuring a torque according to claim 23 or 24, characterized in that the elastically deformable beams (4a) and the substantially stress-free beams (5) are each four in number.  26. A device for measuring a torque according to claim 7, characterized in that the elastically deformable means associating the deformable outer ring (3a) the inner ring (2) is a torsionally deformable tube (4b).  27. A device for measuring a torque according to claim 26, characterized in that the torsionally deformable tube is perforated longitudinally, openings extending in the direction D of the driven shaft separating deformable beams in torsion bending .  28. A device for measuring a torque according to claim 26 or 27, characterized in that the substantially unstressed means associating the second outer ring (3b) with the inner ring (2) is a beam (5) s 'extending radially from the inner ring (2) to the deformable outer ring (3a).  29. A device for measuring a torque according to claim 28, characterized in that two radial beams (5) arranged along a diameter of the substantially unstressed outer ring (3b) connect it to the inner ring (2 ).  30. A device for measuring a torque according to any one of claims 7 to 29, characterized in that the deformable ring (3a), the substantially unstressed outer ring (3b) and the inner ring ( 2) came integrally with their connecting elements (4a, 4b, 5).  31. A device for measuring a torque according to claim 7, characterized in that the elastically deformable means associating the first deformable outer ring (3a) and the inner ring (2) is in the form of a serpentine comprising at least one elbow.  32. A device for measuring a torque according to claim 31, characterized in that it comprises several elastically deformable coils extending radially from the inner ring (2) towards the first deformable outer ring (3a).  33. Application of a device for measuring a torque as presented in claims 1 to 32 to power steering devices of a motor vehicle.